Conventionally, electric motor shafts with an eccentric cam, such as those used in antilock braking systems, are cast, forged or machined in a single piece. A typical electric motor shaft of this type is either forged, cast or machined out of a solid bar stock of alloy steel. Such a manufacture must be used because the shaft must subsequently be hardened and ground so that the finished shaft includes a larger diameter shank portion and a smaller diameter cam pin.
These one-piece shafts of the type used for anti-lock braking systems require a stress curve ground into the transition zone between the larger diameter shank portion and the smaller diameter cam pin. The purpose of this stress curve is to minimize the stress riser that occurs when the transition zone is subject to loading during application of the vehicle brakes. Since these electric motor shafts are a critical component in the anti-lock braking system, failure thereof could be the direct cause of loss of life and other disastrous consequences. Thus, it is particularly important that electric motor shafts used for this application, as well as for many others, be completely reliable.
The stress riser is inherent in the one-piece shaft because of the transition between the larger diameter shank portion and smaller diameter cam pin. Thus, the transition zone becomes a natural weak point in prior art one-piece shafts, and preventing failure in this transition zone has become an important design consideration driving the size, configuration, and material used to fabricate the shafts. Since the stress riser is inherent to the design and can only be minimized, the vehicle industry has developed specific reliability tests to assure shaft integrity and eliminate bending or breaking of the cam pin. For example, in order to pass the Static Flexure Test (Bend Test), the cam pin is inserted into a mating hole in a metal plate of the testing apparatus. A static load of 90 nm is then applied to the shank portion. The cam pin must not bend or break at this minimum load. In order to pass this test, one-piece motor shafts must be of sufficient diameter and incorporates an appropriate stress curve.
Similarly, the Impact Test (energy absorption) subjects a rigidly locked motor shaft to a 70 pound weight dropped from a height of 32 inches so that the weight impacts the cam pin. The testing apparatus records the energy required to break the cam pin off the shank portion of the shaft. Again, the stress curve at the transition zone is the area of vulnerability in this test, and any failure will inevitably occur in this area.
However, while the cam pin requires alloy steel and heat treating to provide a hard, nonwearing inner race surface for the needle bearings, the shank portion of the shaft can remain relatively soft. Hence, using relatively expensive alloy material to form the shank portion of the shaft represents a significant and unnecessary cost to the manufacturer.
Thus, the current one-piece motor shaft has at least two distinct disadvantages; first, it wastes relatively expensive material in the shank portion which is not subject to the same high bearing stresses as the outer surface of the cam pin; secondly, it inherently has a zone of transition from greater to lesser diameter that is particularly vulnerable to breakage.
It would be advantageous for an electric motor shaft such as is used in the anti-lock braking systems of motor vehicles to be designed to eliminate one or both of these undesirable qualities.